Method of and solution for the surface treatment of aluminum



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METHOD OF AND SOLUTION FOR THESUREACE TREATMENT OF ALUMINUM No Drawing. ApplicationAugustS, 1954 Serial No. 448,164

2Claims. (Cl. 252-389) This invention relates to the art of increasing the corrosion resistance of aluminum and alloys thereof in whichaluminum is the principal or predominant ingredient. In the following disclosure the word aluminum is to be understood as including such alloys.

As is Well known, unprotected aluminum surfaces are susceptible to corrosion and in an effort to overcome this tendency it has been customary in the art to deliberately produce anoxide film on .the aluminum surface which, if properly formed by either chemical or electrical means, will serve as an effective :barrier between the basic metal and the corrosive influences of the atmosphere. However, processes of the kind just referred to are open to certain objections. For instance, chemical processes for the production of an oxide film usually produce a film which has a distinctly mottled appearance. Therefore, where it is necessary or desirable to preserve a uniform metallic'appearance, electrochemical treatments have been resorted to because "of their ability to yield a protective oxide film whichdoes not necessarily sub stantially alter the physical appearance of the surface. Such electrochemical processes, however, have the disadvantage of requiring large amounts of electrical equipment. In addition, they are time consuming and relatively expensive.

With the foregoing in mind, the principal object of our invention is the provision of a method and solution for the treatment of aluminum surfaces whereby they may be rendered highly passive and corrosion resistant without discoloring or in-any way marring the surface and without using cumbersome electrolytic equipment.

' Another object of the invention is to provide a process of the character described which is so simple and effective that it can be used by relatively unskilled personnel and under a-wide variety of conditions-4n other words, .a process which is relatively uncritical as to the manner and means by which it is carriedout.

Still another object ofthe invention is the provision of a process for effecting the foregoing objectives without the necessity of removing from .the surface of the aluminum any pre-formed naturally occurring oxide film which may be present.

As indicated, our improved process is in the nature of a passivating procedure which does not appreciably alter the appearance of the aluminum surface and how the foregoing objects together with such other objects as may appear hereinafter or are incident to our invention are attained will now be described.

We have discovered that aluminum surfaces can be passivated and their corrosion resistance greatly increased, and this without marring or discoloring the surface, if they are treated with dilute aqueous acid solutions containing as their principal and essential ingredients some fluorine-bearing compound and a source of hexavalent chromium in accordance with the following teachings.

By the term fluorine-bearing compound we mean a complex fluoride such as fluosilicic acid, fluoboric acid, fluozirconic acid, fluostannic acid,fiuotitanic acid and p rates Patent f 2,825,697 r Ce Patented :Mar. e4, 1958 the soluble saltstofany of :them. 'By the term .hexavalent chromium, we mean chromic acid (CrO or its soluble salts such as ammonium dichromate, potassium dichromate, sodium chromate, etc.

The amounts of these ingredients present .in the solution is important. Of the fluorine-bearing compound there should be present in the bathbetween 0.1 and 1.0 gram per liter calculated as-fluorine. At concentrations of less than 0.1 gram per liter there is a marked falling off in the-elfect produced by the solution. At concentrations greater than -1 gram perliter there is atendency to produce a visible coating upon the aluminum.

As for the quantity of hexavalent chromium in .the bath there should be from 0.4 gram to-4/gramsper liter calculated as CrO Atconcentrations ofless than 0.4 gram very little .effect is produced and when the concentration exceeds 4 grams .per liter there is ,a tendency ,for the solution to produce a visiblecoating on the aluminum.

The pH of the treating solution is also important. Preferably the fluorine-bearingcompound and ;the hemvalent chromium should be present in the solution in such proportions and amounts as to yield a pH of from 2.0 to 4.0. At pHs below 2.0 there is a tendency for the solution to produce visible coatings on the aluminum and at pHs above 4.0 the surface does .not appear to become particularly passive. If the sources of fluorinebearing compound and of hexavalent chromium used'in preparing thesolution are of such nature that the solution does not have. a pH as specified .it may be adjusted,,if too high, by the addition of a suitable .quantity .of some mineralacid such as nitric. acid ,or hydrochloricacid. on the other hand, if .thepH is too .low, it can be raised ,by proper additions ofan alkali such as sodium hydroxide, ammonium hydroxide, etc.

.As for the time of treatment, the temperature of the bathand the manner in which the solution is contacted with the-surface, we have found no particular limitations that seem to be important. A minimum treating time is usually in the neighborhoodof one minute but longer periodsof treatment havealso been employed with excellent results. In general, it .would appearthat best results are to beattained by allowing thesolution to act upon the surface for from one to two minutes where .the temperature of the bath is maintained .at, approximately F.

:However, the matter of temperaturedoes not appear .to bein .any way critical because we have obtained ex- ,cellent results anywhere from ordinary living-room temperatures (i. e., approximately 68 to 78 F), all the way up to. practically theboiling pointof the solution.

The manner in .whichthe solution is broughtinto contact with the surface is also capableof considerbale variation .withoutin any way hindering the results to be ob tained. For example, =the article to be treated maybe immersed in the bath or the solution may be applied to the surface as a spray or it may be wiped with a cloth dampened with the solution or by any other means which insures through wetting of the surface. Where the solution is applied by some wiping expedient it is allowed to dry on the surface but regardless of these various factors we have learned by experience that best results are generally obtained if the following rule of thumb method of preparing the treating bath is employed. When the concentration of fluorine-bearing compound is relatively low then the concentration of hexavalent chromium should also be relatively low and, conversely, when the concentration of fluorine-bearing compound is high the concentration of hexavalent chromium also should be relatively high. Another general rule which it is useful to observe is that where the solution is used hot we prefer a more dilute solution and, conversely, where the solution is to be applied at a relatively cool temperature,

i. e., room temperature, then a more concentrated solution is preferred.

By way of specific example within the teachings above set forth, suitable treating solutions useful in the process of the present invention are as follows:

Example 1 Grams Chromic acid 0.46 Potassium zirconium fluoride 0.29 Water, to make 1 liter.

Example 2 Grams Chromic acid 2 Potassium titanium fluoride 1.4 Water, to make 1 liter.

The material of Example 1 is ideally adapted to treat aluminum in an immersion process and we have secured excellent results by immersion for from one to two minutes with the temperature of the bath maintained at approximately 160 F. Following this, the article is removed, excess liquid is rinsed otf with ordinary tap water and the surface dried.

It should be pointed out that the surface to be treated should be relatively clean although it is not necessary to remove any natural light oxide film which may be present unless such film is of a nature which is aesthetically unappealing or unsatisfactory, it being understood in this connection that the present invention is primarily concerned with a passivating procedure which leaves the surface substantially colorless or unchanged. In other words, naturally occurring oxide films are not deleterious, per se, which, of course, is a distinct advantage over other procedures heretofore employed which first require the removal of any natural oxide film before the treatment is applied. As an actual matter of fact, for some reason which we do not fully understand, the amount of passivation and corrosion resistance secured by our invention seems to be markedly enhanced in situations where a naturally occurring oxide film is left on the surface at the time that our solution is applied thereto.

The material of Example 2 is ideally adapted for application to the surface by means of brush, rags, etc. and we have found that by using a material such as exemplified by Example 2, exceedingly good passivation is achieved if the solution is permitted to act upon the surface for from one to two minutes at room temperature although no harm is done if the material is permitted to dry upon the surface should this be desirable or convenient. However, if the solution is permitted to dry on the surface the aluminum may be stained somewhat due to the hexavalent chromium powder which will be found lying loose on the surface.

Spraying procedures for applying the solution may also be employed but in such procedures as well as in the foregoing procedures length of time and the temperature of the bath do not seem to be too critical although from the standpoint of economy in cost reasonably short 4 periods of exposure to the effects of the solution and relatively low temperatures are to be preferred.

Somewhat surprisingly, the invisible passivation which we secure is obtained by utilizing what may be described as an activating solution which definition is employed because of the peculiar properties imparted to the aluminum surface by the process which we have developed. For example, if a previously deoxidized aluminum surface is treated according to our invention it will exhibit, immediately after the treatment, an extremely low electrical surface contact resistance. This bespeaks an activated surface toward oxidation. However, the electric contact resistance of a surface treated in accordance with our invention rapidly increases after a short time lapse and at a much greater rate than it does on a deoxidized but otherwise untreated aluminum surface. Surfaces treated in accordance with our invention, despite their apparent lack of any visible protective coating, have successfully withstood 168 hours exposure to a salt fog test conducted in accordance with U. S. Government specification QQM15 1a. As contrasted with this showing, previously deoxidized but otherwise untreated aluminum surfaces even after a lapse of time during which a naturally occurring oxide film will form, exhibit far less corrosion resistance when exposed to the same salt fog test, a typical example being approximately 24 hours before substantial break-down occurs.

We claim:

1. The method of increasing the corrosion resistance of an aluminum surface without changing its appearance which consists in treating the surface with an aqueous acid solution consisting essentially of hexavalent chromium and a soluble complex fluorine-bearing compound, the amount of hexavalent chromium being from 0.4 gram to 4 grams per liter calculated as CrO and the amount of fluorine-bearing compound being from 0.1 gram to 1.0 gram per liter calculated as fluorine and the pH of the solution being from 2.0 to 4.0.

.2. The method of claim 1 wherein the hexavalent chromium is chosen from the group consisting of chromic acid (Cr0 and its soluble salts and the fluorine-bearing compound is chosen from the group which consists of fluosilicic acid, fluoboric acid, fiuozirconic acid, fluostannic acid, fiuotitanic acid and the soluble salts thereof.

References Cited in the file of this patent UNITED STATES. PATENTS 2,494,910 Spruance Jan. 17, 1950 2,499,231 Stareck Feb. 18, 1950 2,563,431 Spruance Aug. 7, 1951 2,568,936 Spruance Sept. 25, 1951 2,655,439 Floersch Oct. 13, 1953 2,678,291 Spruance et a1. May 11, 1954 2,727,841 Chester Dec. 20, 1955 2,762,731 Heller Sept. 11, 1956 2,798,829 Newhard et a1. July 9, 1957 2,798,830 Newhard et a1. July 9, 1957 

1. THE METHOD OF INCREASING THE CORROSION RESISTANCE OF AN ALUMINUM SURFACE WITHOUT CHANGING ITS APPEARANCE WHICH CONSISTS IN TREATING THE SURFACE WITH AN AQUEOUS ACID SOLUTION CONSISTING ESSENTIALLY OF HEXAVALENT CHROMIUM AND A SOLUBLE COMPLEX FLUORINE-BEARING COMPOUND, THE AMOUNT OF HEXAVALENT CHROMIUM BEING FROM 0.4 GRAM TO 4 GRAMS PER LITER CALCULATED AS CRO3 AND THE AMOUNT OF FLUORINE-BEARING COMPOUND BEING FROM 0.1 GRAM TO 1.0 GRAM PER LITER CALCULATED AS FLUORINE AND THE PH OF THE SOLUTION BEING FROM 2.0 TO 4.0. 